A variety of hydraulic systems are used in oilfield service equipment. Hydraulic systems often utilize a hydraulic pump coupled to and powering a hydraulic motor used to drive specific well equipment. For example, the hydraulic systems can be employed in well cementing operations, well stimulation operations, and coiled tubing services to drive centrifugal pumps, high-pressure reciprocating pumps, coiled tubing injector heads, and other types of equipment. The hydraulic systems often are used in important applications and their dependability can have a direct impact on the success of the well service. However, there are no adequate methods or devices for monitoring operation of the existing hydraulic systems to determine potential failure conditions.